The present disclosure relates to a fuser device such as a photocopier, printer, fax machine, a multifunction machine combining these, or the like; and to an image forming apparatus provided with the same; and, in particular, to a fuser device capable of switching between a state in which pressure is applied to a fuser nip and a state in which pressure upon the fuser nip is released and to an image forming apparatus provided with the same.
There are known in the art roller-type fuser devices, which are provided with a heating roller and a pressure roller that rotate in contact with each other, and belt-type fuser devices, which are provided with an endless heating belt as a heating member. In a belt-type fuser device, for example, heat and pressure are applied to a toner image carried upon a recording medium at a nip between the heating belt and a pressure roller, which are pressed together, fusing the toner image to the recording medium.
Related technologies for enabling the pressure applied at the nip formed by the heating belt and pressure roller of such a fuser device to be varied are known. In a fuser device according to a first related technology, a spring is interposed between the heating belt and the pressure roller, applying a predetermined pressure to the nip. The state of pressure between the heating belt and the pressure roller is capable of being varied through the operation of a pressure releasing member by a variable pressure member receiving motive force from a motor. A sensor detects whether there is a state of pressure between the heating belt and the pressure roller through the upward and downward motion of a flag provided on the pressure releasing member. When the motor is rotatably driven, the pressure releasing member is moved to a predetermined position by the variable pressure member, the state of pressure between the heating belt and the pressure roller is released, and the sensor detects the flag of the pressure releasing member. This configuration allows jammed paper to be removed from the fuser device when the recording medium causes a jam at the nip. Furthermore, the pressure is released at all times other than when an image is being formed, whereby the heating belt and the pressure roller are prevented from being deformed by the pressure exerted by each upon the other.
In certain fuser devices, joule heat is produced by an excess current generated in an inductive heat-generating layer provided on the heating belt by a magnetic field generated by an inductive heating member, causing the heating belt to generate heat via electromagnetic induction. In such fuser devices, the heating belt has low heat capacity. Therefore, there is a risk of the heating belt breaking from excessive heating when the heating belt is heated while stopped. To prevent the heating belt from breaking, the rotation of the heating belt is detected, and heating of the heating belt is stopped when the heating belt is not rotating.
A fuser device according to a second related technology is provided with, for example, an inductive heating member for heating a heating belt by electromagnetic induction disposed facing a fuser roller with the heating belt interposed therebetween, and a rotation detector for detecting the rotation of the heating belt. The rotation detector has a rotation detector plate that rotates integrally with a roller rotatably driven by the fuser roller and has a portion of the circumference thereof cut out, and a photosensor provided with a light-emitting part and a light receiving part disposed on either side of the rotation detector plate. When the heating belt rotates, light from the light-emitting part of the photosensor is detected by the light receiving part every time the cutout of the rotation detector plate passes the light receiving part of the photosensor, thereby allowing the rotation of the heating belt to be detected.
The fuser device according to the first related technology described above is provided with a sensor for detecting whether there is a state of pressure between the heating belt and the pressure roller, and the fuser device according to the second related technology is provided with a photosensor for detecting the rotation of the heating belt. In a fuser device in which the state of pressure between a heating member such as a heating belt and a pressure-applying member such as a pressure roller is variable, it is necessary to have a sensor for detecting whether there is a state of pressure between the heating member and the pressure-applying member, as well as a sensor for detecting the rotation of the heating member; however, a problem is presented in that when, for example, a sensor for detecting the upward/downward motion of a flag as disclosed in the first related technology and a photosensor for detecting the rotation of a rotation detector plate as disclosed in the second related technology are provided separately, the size of the device increases, and manufacturing costs go up.